1. Field of the Invention
The present invention relates to a sample holding control technology in an electrostatic chuck of a charged-particle-beam apparatus.
2. Description of Related Art
An apparatus for processing a semiconductor wafer, an apparatus for inspecting a circuit pattern on a semiconductor wafer, or the like using a charged-particle beam has a sample holding function by an electrostatic chuck method using an electrostatic-clamping force in order to hold the semiconductor wafer.
An example of such an apparatus is an electron-beam-based length measurement apparatus. This apparatus is intended to measure a width or a position of a circuit pattern fabricated on a semiconductor wafer. The electrostatic chuck method includes the Coulomb method and the Johnsen-Rahbek method. In the electron-beam-based length measurement apparatus, the Coulomb method in which no current flows in the semiconductor wafer is adopted since the probe current is very small.
The Coulomb method needs a larger applied voltage compared to the Johnsen-Rahbek method, because a resistivity of a dielectric is very large. This causes problems described later.
As for the method for determining whether or not the semiconductor wafer (sometimes called “wafer” in this application) is normally clamped by the electrostatic chuck, there is proposed an method which sets as a reference value, the current flowing through the electrostatic chuck circuit when the wafer is clamped normally, and determines that the wafer is not correctly disposed (abnormal) when the current is smaller than the reference value (see Patent Document 1, for example).